Commercial shipping has become one of the largest sources of air pollution in Europe. As a result, the International Maritime Organization (IMO) has tightened its requirements for reduction of emissions of sulfur oxides (SOx) and particulate matter (PM) (see for example the 59th session of the Marine Environment Protection Committee, 16 Jul. 2009). One way of reducing oxides of sulfur is to use low-sulfur fuels. However, the cost of such fuels is more than current marine fuels and when consumed at the rate that ships require, introduces significant costs to the ship owners. Fortunately, IMO regulations allow the use of alternative technologies to reduce SOx emissions from conventional marine fuels.
Absorption involves bringing contaminated effluent gas into contact with a liquid absorbent so that one or more constituents of the effluent gas are selectively dissolved into a relatively nonvolatile liquid.
Scrubbing effectiveness relates to Henry's Law: the mass of a gas that dissolves in a definite volume of liquid is directly proportional to the pressure of the gas:                P=Hx        x is the solubility of a gas in the solution phase        H is Henry's constant        P is the partial pressure of a gas above the solution.        Dissolving a gas in a liquid is usually an exothermic process. Therefore, lowering the temperature generally increases the solubility of gases in liquids.        
SOx gases (in particular SO2) are formed when fuels that contain sulfur are burned. SO2 dissolves in water vapour to form acid and interacts with other gases and particles in the air to form sulfates and other products harmful to people and the environment (e.g., sulfurous smog and acid rain). SO2 solubility in water rises steeply as the temperature of the water-gas mixture decreases.
Absorption systems are designed to transfer the SO2 from a gas phase to a liquid phase, which is accomplished by providing intimate contact between the gas and the liquid, which allows optimum diffusion of the gas into the solution. Without being bound by theory, the mechanism of removal of a pollutant from the gas stream takes place in three steps: 1) diffusion of the pollutant gas to the surface of the liquid, 2) transfer across the gas-liquid interface, and 3) diffusion of the dissolved gas away from the interface into the liquid.
The transfer of a substance from one phase (gas) to another phase (liquid) requires time. The rate of transfer is proportional to the surface of contact between the phases, the resistance to the transfer, and the driving force present for the mass transfer, and can be represented by the following formula:Rate of Transfer=(Driving Force)×(Area available for Transfer)/(Resistance to Transfer)
Any increase in the transfer rate leads to a more compact mass transfer device that is generally more economical.
The Driving Force is the chemical potential of the substance to be transferred. Every substance has a chemical (or “mass”) potential which drives it from one phase to another. The value (Y1−Y2) is the difference in concentration (or driving force) in the gas phase. The value (X1−X2) is the difference in concentration (or driving force) in the liquid phase
The required gas mass transfer (i.e., Y1−Y2) in the system can be expressed by the required number of gas mass Net Transfer Units (NTUs). The required number of NTUs for the system can be determined as follows:                Heavy Fuel contains 3.5% sulfur concentration or 35,000 ppm        When fuel is burned in an engine, approximately 1,000 ppm of the 35,000 is converted to SO2 gas (the rest of the sulfur is not combusted and is attached to particles, etc.)        The sulfur gas concentration fed into the exhaust cleansing system is then 1,000 ppm        Hence, Y1=1,000 ppm        The required Y2 to meet MARPOL Annex VI regulations is approx <=30 ppm        Hence, Y2=30 ppm        Giving Y1−Y2=1000−30=970 ppm        
In logarithmic form we have:                Ln(Y1−Y2)=Ln(Y1)/Ln(Y2)=Ln(Y1/Y2)=Ln(1000/30)=3.50        The value 3.50 is defined as the required number of gas mass transfer units that is required to be removed (i.e., SO2 gas mass transferred from gas to liquid state)        NTU-R=Net Transfer Units Required by System=3.50        
Thus, NTU-R can be viewed as the required gas mass transfer efficiency of the system.
As the gas transits the system over time “t0” to time “tn” the SO2 gas concentration must be reduced from 1,000 ppm to 30 ppm. In other words, the system should have a gas mass transfer capability of 3.50 NTUs, i.e. Ln(Y1−Y2)=Ln(Y)/Ln(Y2)=Ln(Y1/Y2)=Ln(1000/30)=3.50. As the exhaust gas travels through the cleaning reactor system, the SO2 is absorbed at different rates and at different efficiencies; the reason is that the Driving Force (chemical potential) of the SO2 substance to be transferred from the gas to liquid decreases.
In constructing a gas cleaning system with a number of gas cleaning zones, the zones can be arranged in series. For example, the exhaust gas transits the cleaning zones over time t0 to tn. Each zone has its own cleaning capability (i.e., efficiency) which is measured by the Zone's NTU value; the zone efficiency depends on SO2 concentration, temperature, surface contact area, contact time within the zone, etc. The sum of capabilities for all zones must equal the required gas mass transfer capability which in this instance is Sum of NTU for all Zones=3.50
Adsorption is a mass transfer process that involves passing a gas stream through the surface of prepared porous solids (adsorbents). The surfaces of the porous solid substance attract and hold the gas by physical or chemical adsorption. In other words, adsorption is the bonding of two particles or molecules. An adsorbent may be any solid material which provides bonding sites. In the case of a gas cleaning system, the particulate matter particles provide the bonding sites. An adsorbate may be any dissolved molecular substance or particles in suspension. In the case of a gas cleaning system, there are three adsorbates (i.e., three types of things that are being bonded to the adsorbent):                1. SO2 molecules in the gas steam are being bonded to particulate matter (PM) particles        2. VOC (Volatile Organic Compounds) are being bonded to PM particles        3. Small PM particles are being bonded to larger PM particles to form particle clusters.        
Current technologies for reducing SOx emissions from flue gases include wet scrubbers using sodium hydroxide solutions. However, equipment employing these technologies is large and cumbersome and cannot be deployed easily in the limited confines on board ships. Other factors limiting a wet scrubber for use on board a ship include weight and electrical power limitations.
U.S. Pat. No. 7,018,451 discloses a method for removing sour gas and acid gas components from gas mixture by absorption using a solvent or reagent and turbulent mixing of the gas with the solvent or reagent.
U.S. Pat. No. 7,273,513 discloses a method for simultaneously absorbing selected acid gas components from a gas stream and flashing off hydrocarbons entrained in a liquid stream including a solvent or reagent, wherein the reagent is an amine.
U.S. Pat. No. 8,524,180 discloses a method for removing particulate matter from diesel exhaust gases using water or an aqueous solution of bases or salts with a minimal drop in exhaust pressure.
U.S. Application Publication 2013/0213231 discloses a double-pipe apparatus and method for scrubbing flue gases using cyclonic action and fans to increase the flue gas velocity through the apparatus. The flue gas scrubbing is achieved using a fluid comprising an aqueous basic solution.
Thus, there remains a long-felt need for a wet flue gas scrubber that is efficient, compact, and of straightforward design, in particular, one that meets these technical requirements while being sufficiently compact to be used, e.g., on board a ship.